1. Field of the Invention
The present invention relates to a method of controlling an apparatus for sequentially heat treating a series of substrates, such as semiconductor wafers, in a furnace or the like.
2. Description of the Prior Art
To manufacture semiconductor wafers, heat treatment is performed in various steps. During such heat treatment, it is very important to control the temperature of the furnace into which the wafers are introduced.
Japanese Patent Laying-Open Gazette No. 160512/1987 discloses an example of a preferable method of controlling the temperature of such a furnace. In this method, wafers are heated by radiation in a furnace which is controlled in an open-loop manner according Stefan-Boltzmann's law. With such a method, temperature for heat treatment can be controlled with considerable accuracy while a series of wafers are periodically and sequentially introduced into the furnace to be thermally treated.
However, highly accurate temperature control cannot always be attained with the known method, especially during heat treatment of the first of a series of wafers, that is, when power is supplied to a furnace while the furnace is at room temperature, or when heat treatment is resumes after an interval of non-use. This is because power is supplied to the furnace on the premise that the furnace is always in some constant state at the start of the heat treatment of each wafer. However, the state of the furnace during a starting operation or a re-starting operation is different. Therefore, the first several wafers heated during a starting operation, or during a restarting operation, must be disposed of as defective because of inaccurate temperature control. As a result, the yield of wafers is undesirably reduced.
To solve this problem, the following two techniques have been proposed In the first technique, a dummy wafer is introduced into a furnace simultaneously with a product wafer which is to be actually subjected to heat treatment. The product wafer is hereinafter referred to as "object wafer". The temperature of the dummy wafer is monitored through a thermocouple or the like and the furnace is controlled in a closed-loop manner according to the temperature of the dummy wafer.
In the second technique, the furnace is controlled in a closed-loop manner on the basis of the temperature of the object wager, as monitored by a radiation thermometer.
In the first technique, however, since the dummy wafer is introduced into the furnace separately from the object wafers, an additional device for handling the dummy wafer is required. Further, since the temperature of the dummy wafer is monitored to indirectly determine the temperature of the object wafers, the size of the dummy wafer is identical to the size of the object wafers. This makes the handling device fairly large. Therefore, the apparatus as a whole is inevitably increased in size and complicated.
The second technique does not have the problem of the first technique because the temperature of the object wafers is measured directly by the radiation thermometer. A dummy wafer is not utilized. However, since thermal radiation from the casing of the furnace is reflected by a surface of an object wafer, thermal radiation incident upon the radiation thermometer, which is directed to the object wafer, includes not only thermal radiation from the object wafer itself but that from a quartz tube forming the furnace casing. Thus, the radiation thermometer must be calibrated in advance in a normal cycle operation where the object wafers are periodically and sequentially treated. In spite of the calibration, however, considerable errors are caused in the starting operation and the re-starting operation because of the low temperature of the quartz tube. Consequently, temperature information from the radiation thermometer is not always reliable. In other words, heat treatment control does not always shown high reproducibility. Further, the reproducibility of the heat treatment control is significantly reduced when the radiation thermometer is installed toward a rear surface of the semiconductor wafer because the rear surface is satinfinished and likely to irregularly reflect thermal radiation from the quartz tube.
Incidentally, the term "substrate" in this specification refers to any kind of substrate such as a semiconductor substrate (a semiconductor wafer), a ferrite substrate for magnetic recording or the like.
Accordingly, an object of the present invention is to provide a control method which can improve reproducibility in heat treatment of substrates when starting and re-starting operations without increasing the size or complexity of the heat treatment apparatus.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.